1. FIELD OF THE INVENTION
This invention relates to a power assisted braking system which incorporates a power steering unit and more particularly to a servo motor for same which is fail safe in that it is equipped with a pressure reservoir which can be temporarily used in lieu of the pump should the latter fail.
2. DESCRIPTION OF THE PRIOR ART
As is well known automotive vehicles often utilize a single source of hydraulic fluid under pressure to operate the power brake system as well as other fluid power operated vehicle systems such as the power steering system.
It is also known to apply the brakes directly with the hydraulic fluid under pressure supplied by the pump directly to the brake cylinders of the brakes to initiate the operation of same via a suitable valving system without the interposition of a master cylinder. This system while effective suffers from the drawback that should the supply of fluid under pressure for some reason be cut off due to a malfunction the entire braking system becomes inoperative, and the associated systems such as the power steering lose a great deal of their effectiveness.
Other systems which overcome the above mentioned defect utilize a servo motor which is interposed between the master cylinder of the braking system and the brake pedal or a mechanism directly connected thereto, whereby should the pump fail to supply adequate hydraulic pressure the construction of the servo motor permits direct mechanical operation of the master cylinder. One inherent defect found in all such arrangements where the servo motor is interposed between the master cylinder and the brake is that upon loss of the supply of pressurised fluid from the pump associated therewith, the manual operation of the brakes requires greater physical effort than a normal unassisted braking system. Simultaneously a sudden loss of effective operation of the other power assist system combined with the braking system (fed pressurised fluid from the same pump) is experienced.
To overcome the above mentioned problem it has been proposed to equip the system with a pressurised fluid accumulator, similar to those usually associated with the systems wherein the master cylinder is omitted. However these systems have suffered from undesirable complexity and slowness in changing the supply of pressurised fluid from the pump to the accumulator.
Thus there still remains a need to provide a combined power braking and steering system which is simple in construction, relatively light and preferably comprises several small units interconnectable by conduiting, facilitating deployment within the motor compartment or the like and which is instantly responsive to a loss of fluid pressure from the pump to supply in lieu thereof the accumulated pressurised fluid.